The present invention relates to lubricating greases. In particular the invention relates to lubricating greases providing improved performance over a wide range of temperatures. Most especially it provides improved performance greases for use in the lubrication of high speed bearings.
Modern advances in technology and especially in the field of aerospace require that equipment be capable of operation over an ever increasing range of temperatures. For example, equipment installed in an aircraft may be expected to stand under Arctic conditions for a period of time and then to commence to operate immediately on demand. This same equipment may then be required during subsequent use to operate at running temperatures up to 200.degree. C. For example, the efficient use of various machines, mechanisms and instruments in aircraft frequently calls for a degree of manipulative skill and consistent performance that cannot be directly achieved by human operators. Recourse is therefore made to automatic control systems, which are actuated by the difference between the actual behavior of the system and the desired behavior and into which some source of external power is introduced. Such so-called servo systems are therefore said to be error actuated and power amplifying. An important requirement is the accuracy of regulation or follow-up. This necessitates the use of closed loop sequence control and is effected by introducing a feed-back link between the output and input of the system. A servomotor is one form of element commonly included to provide the requisite precision and speed of response.
In the present context, a servomotor is a small two-phase a-c motor. One phase has constant excitation on the field coil and the other is fed from the error signal via an amplifier, which raises the power level to the point required by the motor. A servomotor has the characteristic of being reversible and its speed is proportional to the applied voltage. Thus, the corrective action of the servomotor is proportional to the error, i.e., to the divergence from the desired output of the servo system.
It follows that a servomotor is usually operating at well below maximum speed. Its speed only surges when a change is made in the input signal. As soon as the error signal has been corrected, the motor speed falls and the sermomotor operates in a low-speed hunting mode until a further change in the input signal is made.
Some servomotors are installed in the hot environment of engine bays, where typically, they are used to control throttle mechanisms and constant-speed drives. Thus, the bearings employed in these servomotors often operate as speeds up to and in excess of 6,000 revolutions per minute (r.p.m.). Greases for use in such bearings must have very low starting torques at low temperatures and yet have very good high temperature stability.
The greases of the present invention are intended primarily for use in bearings operating at speeds up to and in excess of 6,000 revolutions per minute (r.p.m.) at temperatures from 200.degree. C. down to minus 54.degree. C.
Under these conditions, a lifetime in excess of six months (4,320 hours) at 200.degree. C. is considered desirable.
In order that a grease formulation should be considered for the present use, it is essential that it has good low-temperature properties and yet is stable to evaporation and oxidation at elevated temperatures and that it has a good storage or shelf life. In order to define the requirements more specifically, greases for consideration can be evaluated using the following suggested scheme.
(1) The grease should preferably be an NLGI Grade 2 grease. NLGI grading is that of the National Lubricating Grease Institute and the grades are defined as ranges of the 60 double stroke worked penetration at 25.degree. C. as determined by the method of IP 50/69 ASTM D 127-68) "Cone Penetration of Lubricating Greases". An NLGI Grade 2 corresponds to an IP penetration at 25.degree. C. between 265-295.
(2) The dropping point to the method of IP 132/65 (ASTM D.566-76) should preferably be in excess of 250.degree. C.
(3) When examined under an optical microscope, there should be no large particles visible.
(4) When submitted to the Low Temperature Torque Test according to the method of IP 186/64 at minus 54.degree. C. the starting torque should be less than 5,000 g.cm., and the running torque less than 2,000 g.cm.
(5) The high temperature stability may be examined using a Thin Film Oxidation Test at 200.degree. C. where a film of grease 63.5 mm.times.35.0 mm.times.1.6 mm is smeared onto a mild steel strip 76.0 mm.times.51.0 mm.times.1.6 mm and maintained at 200.degree. C. in an air oven for a minimum period of 72 hours. Appearance and weight loss are noted. Weight loss should be as low as possible.
(6) The storage life can be investigated by packing bearings, glass containers and/or tins with the grease and examining these during and after storage under relevant conditions. For the present purpose, the conditions were (a) ambient temperature, (b) 40.degree. C., (c) 100.degree. C., and (d) 200.degree. C.
(7) Resistance to evaporation as investigated using the Evaporation Test according to the method of IP 183/63 at 200.degree. C. modified to use a 1 mm layer of grease with both air and nitrogen flow. A very low percentage evaporation is required under all conditions.
Further evaluation of those greases which satisfy the above requirements is then carried out by actual use in typical bearings run under simulated use conditions. Suitable miniature ball bearings can be SR Z RHH 7P5 8LDZD bearings, 0.125 inch bore.times.0.375 inch o.d..times.0.156 inch wide supplied by Miniature Precision Bearing Company. They are manufactured from stainless steel and are fitted with a ribbon container and metal shields on either side of the balls. These bearings can be housed in a suitable test rig and run under realistic conditions. For example, they can be driven by small servomotors of the type employed as described earlier and mounted within an oven regulated at some chosen temperature, for example 200.degree. C. Failure occurs when the rig driving motor no longer rotates the bearing.
An alternative rig test procedure is Ministry of Defense method DEF-2000, Method 27, Procedure B, called the Pope Rig Test in which a test bearing is rotated under light radial and thrust loads at 10,000 revolutions per minute until either a specified time is reached or until failure occurs. Bearings normally used in this test are SAE 204 (8-ball 20 mm bore) fabricated from 18-4-1 high speed or M 10 tool steel, tempered for use up to 370.degree. C., manufactured to ABEC 3 standards and having a radial clearance of 0.0010 to 0.0012 inch. Ball retainers are fabricated from heat treated silver plated beryllium copper capable of withstanding temperatures up to 370.degree. C. We did not, however, employ the high quality, expensive bearings specified for this test in the present work but replaced them by SAE 204 (8-ball, 20 mm bore) fabricated from 52100 steel and reported to be heat treated to 177.degree. C. These bearings have previously been found to give adequate service at 200.degree. C. The bearings are manufactured to ABEC 3 standards and have a radial clearance of 0.0006 to 0.0010 inch. The balls are retained by a pressed steel cage. This rig test is best carried out with test conditions controlled on bearing temperature and not oven temperature. Failure occurs when motor power output increases to a value approximately 300% above that for steady running for a period greater than 30 seconds; when belt slippage occurs or when the bearing no longer rotates. Success in this test is an indication of the suitability of a grease for use in more normal size bearings as compared to the miniature ones employed in the previous rig test.
Extensive screening of prior art high performance greases, having a variety of base fluids and thickening systems using the above-suggested evaluation scheme, has shown none to have a lifetime greater than two months (1,500 hours) as well as satisfying the other requirements even when tested at a maximum temperature of only 175.degree. C.